Embodiments of the present invention relate generally to new and useful improvements in control of towed objects, and in particular control of the altitude of a passive towed object.
Towed objects may be divided into two classes, those capable of actively controlling their own attitude, and those with no control surfaces or actuators of their own. Members of the active class include remotely operated vehicles (ROVs), unmanned underwater vehicles (UUVs), and piloted ships being towed. Members of the passive class are just as diverse, and include fishing gear, camera sleds, side scan sonars, and disabled ships and hulks.
Actively controlled ROVs have a well-established set of control methodologies based on conventional control theory. Control techniques are diverse, and include standard proportional, integral, derivative (PID) methods as well as more advanced techniques involving sliding mode control, LQG/LTR, fuzzy logic, and hybrid techniques.
Passive objects, which have no control surfaces or actuators of their own, are generally launched, towed, and retrieved manually by an expert crew. An expert crew, rather than some form of control system, is the state of the art. Traditional control techniques require an accurate, quantitative model of the system to be controlled. Such models normally consist of a set of transfer functions in the s-domain or a set of coupled differential equations. Often, the form of the model is known and the model's parameters must be determined using a system identification procedure. However, models of passive towed objects are normally unavailable. Model data may be proprietary or worse, performing system identification may not be cost-effective even for the equipment's original manufacturer.
Nevertheless, expert crews successfully tow passive objects on a regular basis. However, the current state of the art requires that an expert crew must be present during tow operations. Such a crew must be trained and must practice their skills in order to remain proficient. Furthermore, towing an object for mine hunting purposes places the crew in danger while requiring them to perform a dull and repetitive task. A manned presence also imposes infrastructure requirements—a mine hunting vessel must be large enough to house the crew and support life for weeks at a time.
In short, there exists a need in the art for an automated control system for passive towed objects.